SUZ-4 zeolite has been given the three-letter IUPAC designation code SZR. The framework of the SUZ-4 zeolite consists of 4-, 5-, 6-, 8- and 10-membered rings of 3-dimensional channel systems. It has an ortho-rhombic unit cell with dimensions of a=18.8696, b=14.4008 and c=7.5140 Å, respectively. The 10-membered ring channels of the SUZ-4 zeolite are the main straight channels in the framework, and they are interconnected by zig-zag 8-ring channels. The 10-ring straight channels of the SUZ-4 zeolite have dimensions of 4.6×5.2 Å, i.e. notably smaller than the 10-ring channels found in the ZSM-5 zeolite (5.3×5.5 and 5.4>5.6 Å).
Within the catalyst area, the SUZ-4 zeolite is known to be a selective and stable dehydration catalyst in the process for producing dimethyl ether from methanol (Jiang, S. et al., Chemistry Letters 33, no. 8, 1048 (2004)).
According to U.S. Pat. No. 6,936,562 B2 (General Motors Corp.), certain metal-exchanged SUZ-4 zeolites have been prepared which have catalytic activity in the reduction of NOx in the exhaust gas from a hydrocarbon or alcohol fuelled engine.
Similar hydrothermally-stable catalysts based on substituted SUZ-4 zeolites are described in the related U.S. Pat. No. 6,645,448 B2 (General Motors Corp.), and in U.S. Pat. No. 5,118,483 B2 (British Petroleum Co.) various crystalline forms of the SUZ-4 zeolite based on crystalline (metallo) silicates are described. It should be noted that—with reference to e.g. U.S. Pat. No. 5,118,483—the standard methods for forming SUZ-4 zeolites will usually provide thermochemically-preferred Si/Al stoichiometries, regardless of the molar ratios of the Si and Al components in the starting materials.
EP 0 706 984 A1 (BP Chemicals Ltd.) discloses the catalytic use of SUZ-4 zeolite for the isomerisation of hydrocarbons, and in U.S. Pat. No. 6,514,470 B2 (University of California) a large number of aluminium-silicate materials, including SUZ-4 zeolite, are used as catalysts for lean burn exhaust abatement. JP 2009-233620 A (Tosoh Corp.) describes the use of SUZ-4 zeolite in an SCR catalyst with the objective to give the catalyst improved hydrothermal durability.
Most recently, variants of the methanol to olefins (MTO) process have been disclosed in EP 1 963 241 A2 and in WO 2008/042616 A2 (both to UOP LLC).
One of the main challenges within the field of MTO catalysis is that the known catalysts have a very limited life span, requiring continuous regeneration at elevated temperature which eventually lead to irreversible damage to the catalyst. According to the inventors, this is also the case with the SUZ-4 zeolite-based catalysts, because the life span of the current SUZ-4 zeolite-based catalyst prepared using the standard methods in the MTO catalysis field does not exceed that of other catalysts such as silicoaluminophosphate molecular sieves (e.g. SAPO-34). However, it has now surprisingly been found that the life span of the SUZ-4 zeolite-based catalyst for MTO use can be markedly improved by either modification of the zeolite acidity (i.e. by increasing the Si/Al ratio, partial ion-exchange of alkali counter-ions)